A kernel program of an OS represented by Unix (registered trademark) and Windows (registered trademark) is configured to include a file system from the very nature. The file system is a system having a function of managing files (data) stored in (auxiliary) storage devices such as an HDD (Hard Disk Drive) or an SSD (Solid State Drive). The file system provides to an application program an abstract (access) interface used for operating files while concealing the actual state of storing data in the storage device from the application program.
The file system generally presents the state of data in such a way that all the files constitutes a hierarchical structure under a root directory. The file system is further adapted for using a mount operation to subordinate one directory to another directory, which enables the application program to specify a target file by indicating the directory (hierarchical position) and the file name.
The file system generally has a structure which gives, in every file, access authorities of “read r”, “write w” and “execute e” to each of “owner”, “group” and “other users”. The access authorities are expressed, e.g. as 3 bits per one user, 9 bits in a total, as follows.
“owner”“group”“other users”r, w, er, w, er, w, eAn example of the result of referring to the access authorities by using ls command of Unix (registered trademark) is as follows.                % ls -l owner.c        -rw-r--r-- 1 ikeda toyotsu 375 May 14 10:35 owner.cThis referring result shows that the owner can perform reading and writing however is inhibited from directly doing execution, and that the group members and other users can do reading however are allowed to perform neither writing nor executing. Using such a form, the file system manages the access authorities of each file.        